Yun-Hi Lee

Field emission from 4.5 in. single-walled and multiwalled carbon nanotube films

Field emission properties of 4.5 in. flat panel displays in a diode type panel using single-walled (SWNTs) and multiwalled carbon nanotube tips (MWNTs) were characterized and compared. The panel, fabricated by a slurry squeezing and surface rubbing technique, enables the generation of more emission sites by removing materials on the surface. The turn-on field of MWNTs decreased from 6.4 to 3 V/μm by treatment of the surface, and that of SWNTs also decreased, from 4.5 to 2 V/μm. The density of aligned MWNTs is approximately 2/μm2, whereas the aligned SWNTs were uniformly distributed, with densities of 5–10/μm2. As a result, SWNT films show higher emission uniformity than MWNT films. A gradual degradation over time was observed in both MWNTs and SWNTs. The current stability curve of the SWNTs decreased about 20%, while that of the MWNTs decreased less than 10%.

Multiwall carbon nanotube gas sensor fabricated using thermomechanical structure

We address a multiwall carbon nanotube (MWCNT) gas sensor that has an enhanced reproducibility. The sensor consists of a heater, an insulating layer, a pair of contact electrodes, and an MWCNT-sensing film on a micromachined diaphragm. The heater plays a role in the temperature changes to modify the rates for desorption of gases on the sensing surface during sensor operation. Gas sensor responses of MWCNT-film to NO/sub 2/ at room temperature are reported. We show that the sensor exhibits a reversible response with a time constant of a few minutes at thermal treatment temperature of 130/spl deg/C.

Electron‐paramagnetic‐resonance study of the Mn2+ luminescence center in ZnS:Mn powder and thin films

The number of isolated Mn2+ ions and Mn2+ clusters in ZnS:Mn powder and thin films has been studied using Mn2+ spectra measured at room temperature with an X‐band electron‐paramagnetic‐resonance spectrometer. While the concentration of the isolated Mn2+ ions decreases with increasing Mn concentration, the concentration of the clusters increases. At low Mn concentration, the Mn2+ ion substitutes for the Zn ion in ZnS:Mn in the cubic phase. At high Mn concentrations, where the ZnS powder has a dominant hexagonal phase, the Mn ion still prefers to substitute for Zn in ZnS:Mn at the cubic site rather than at the hexagonal site.

Multiwall carbon nanotube sensor for monitoring engine oil degradation

An engine oil condition sensor based on multiwall carbon nanotubes was fabricated with the screen printing method. Carbon nanotube (CNT) paste was printed between two parallel-aligned electrodes. The conductivity change in the engine oil according to mileage was measured by using CNT oil sensor. The sensor output was compared with total acid number (TAN) to determine the remaining useful life of the engine oil. It was found that the sensor output was closely similar to TAN of the engine oil. As a result, a real-time engine oil condition monitoring sensor can be realized using CNTs as sensitive materials.

IMPACT OF SURFACE PROPERTIES ON THE DIELECTRIC BREAKDOWN FOR POLYCRYSTALLINE AND MULTILAYERED BATIO3 THIN FILMS

The dielectric reliability for polycrystalline and multilayered BaTiO3 thin films has been evaluated using time-zero and time-dependent dielectric breakdown techniques. The histogram of dielectric breakdown for multilayered BaTiO3 thin films showed a typical Weibull distribution in contrast to a random distribution when compared with polycrystalline BaTiO3 thin films. The measurement resulted in that the 400 nm-thick multilayered BaTiO3 thin film sustained about 105 hour-long operation at 1 MV/cm, showing superior properties when compared with polycrystalline. The smaller leakage current level was obtained for a multilayered BaTiO3 film having a relatively thick underlayer of polycrystalline BaTiO3 film. The value of the breakdown field was smaller at the thicker multilayered BaTiO3 while the distribution of the breakdown widened for thicker film. Analysis of the roughness for the films confirmed that the field breakdown mechanism (e.g., lowering and spreading) is related to the surface roughness of the t...

Carrier transport and electron field-emission properties of a nonaligned carbon nanotube thick film mixed with conductive epoxy

We have studied the conduction characteristics of multiwalled carbon nanotubes (MWNTs), which were screen printed in a thick film form for field-emission displays. Resistivity and magnetoresistance were measured as a function of temperature T in the range of 1.7–390 K and magnetic field, respectively. The resistivity of the MWNTs for temperatures of 10–390 K indicates that the system is intrinsically metallic and the resistivity–temperature characteristics are well described by the Mott’s T−1/4 law in temperatures above 10 K, suggesting that the density of states at the Fermi level is constant in the range of 10–100 K. We found that the main contribution to the conductivity comes from carriers that hop directly between localized states via variable-range hopping. The temperature dependence above 10 K is in good agreement with that of an individual multiwalled carbon nanotube. However, below 10 K the resistivity is well fit to Efros T−1/2 law, confirming the presence of a Coulomb gap for the system. With t...

Roughness of ZnS:Pr,Ce/Ta/sub 2/O/sub 5/ interface and its effects on electrical performance of alternating current thin-film electroluminescent devices

Roughness effects of neighboring dielectrics on electrical characteristics of thin-film electroluminescent devices were investigated in order to improve the understanding of physics for the devices. Atomic force microscopy analysis reveal that thicker bottom layer of Ta/sub 2/O/sub 5/ shows rougher surface resulting in the rougher surface of ZnS:Pr,Ce layer. It can be easily seen that the dc leakage current increases rapidly with increase of surface roughness. Furthermore, it is notable that the initiation field of Poole-Frenkel current conduction is lowered by increasing surface roughness of Ta/sub 2/O/sub 5/ thin film. Internal charge-phosphor field (Q/sub int/-F/sub p/) analysis and capacitance-ac voltage (C-V) analysis for ITO-Ta/sub 2/O/sub 5/-ZnS:Pr,Ce-Al and ITO-Ta/sub 2/O/sub 5/-ZnS:Pr,Ce-Ta/sub 2/O/sub 5/-Al show that the steady state phosphor field is smaller and C-V curve in transition region is less steep with increase of root-mean-square roughness between lower dielectric and phosphor layer in the alternating current thin-film electroluminescent (ACTFEL) devices. Therefore, we conclude that interface roughness is one of the physical factors to change the electrical performance of ACTFEL device.

The influence of surface roughness on the electric conduction process in amorphous Ta{sub 2}O{sub 5} thin films

Amorphous Ta{sub 2}O{sub 5} thin films were deposited by radio-frequency magnetron sputtering at the substrate temperatures of 100, 200, and 300 C, respectively. The electrical properties of Ta{sub 2}O{sub 5} thin films were investigated as a function of substrate temperature and film thickness. The leakage current of the Ta{sub 2}O{sub 5} films was in the order of 10{sup {minus}5} to 10{sup {minus}6} A/cm{sup 2} for an applied field of 1 MV/cm. The charge storage capacitances ({epsilon}E{sub breakdown}) were 7.7 (100 C), 7.9 (200 C), and 3.7 (300 C) {micro}C/cm{sup 2}. Most of the electrical analyses were performed with the data obtained for the Ta{sub 2}O{sub 5} thin films deposited at 200 C substrate temperature because they showed optimum electrical properties. The dominant conduction mechanism changed from Schottky emission current at low field to Poole-Frankel current at the high field. With increasing film thickness, the surface roughness increased, whereas the transition fields from the electrode-limited current to the bulk-limited current process decreased. To verify the effect of this surface roughness on the electrical conduction mechanism, a two-dimensional numerical simulator, MEDICI, was used to simulate the electric field distribution at the bulk region of the thin film and the interface region betweenmorexa0» the thin film and electrode.«xa0less

Decrease of the number of the isolated emission center Mn2+ in an aged ZnS:Mn electroluminescent device

We observed a decrease of the number of effective emission centers Mn2+ in the aged ZnS:Mn electroluminescence (EL) devices compared to the fresh EL devices using the electron‐paramagnetic‐resonance technique. Such phenomena can take place during the operation of the EL device, since the isolated Mn can easily diffuse into another site and forming cluster. Another possible explanation is that Mn2+ changes into Mn1+ or Mn3+ by transferring the electronic charge of the isolated Mn2+ to the neighboring Mn ions via sulfur and/or sulfur vacancy. As a result, luminance is lowered due to the decrease in the number of efficient emission centers of isolated Mn2+.

Characteristics of ZnS:Mn thin film electroluminescent device using layered BaTiO3 thin film structures

Abstract A multilayered and a double-layered BaTiO3 thin film structure were applied to the preparation of ZnS:Mn thin film electroluminescent (TFEL) devices. From the characterization of the TFEL devices, it was confirmed that the multilayered BaTiO3 thin film structure prepared by a new stacking method, i.e. deposition of an amorphous layer during continuous cooling of the substrate after the deposition of a polycrystalline layer at higher temperature, had very suitable electrical properties for the insulating layer of the TFEL device. The ZnS:Mn TFEL device using the multilayered BaTiO3 thin film structure showed stable efficiency characteristics with operating time as well as a low turn-on voltage of ∼50xa0V and a high saturated brightness of ∼3000xa0cdxa0m−2.